1. Field of the Invention
The present invention relates to a sheet processing apparatus, and more particularly, it relates to a sheet processing apparatus used with an image forming apparatus such as a copying machine and having a sheet punching device for perforating a sheet to be discharged, and an image forming apparatus having such a sheet processing apparatus.
2. Related Background Art
Conventionally, in some cases, a puncher for forming perforations in an end portion of a sheet (outputted from an image forming apparatus such as a printer, a copying machine and the like) to permit the sheets to be filed in a binder as a sheet bundle has additionally been used with a sheet processing apparatus.
Such a puncher serves to collectively form perforations in a sheet bundle including discharged sheets on which images were formed. In such a puncher, since the sheet bundle must be set in the puncher manually, workability is worsened. Further, if the sheet bundle includes a large number of sheets, a punch and a die of the puncher are subjected to great load, with the result that a greater force is required for perforating the sheet bundle and/or the punch and die are worn to generate poor punching and burrs in the perforations. To avoid this, if the sheet bundle includes a large number of sheets, the sheet bundle must be divided into sheet bundles each including a predetermined number of sheets or less and the divided sheet bundles must be perforated respectively, which results in further reduction of workability.
To cope with this, there has been proposed a sheet processing apparatus in which a puncher is provided in a sheet convey path and sheets stacked (as a sheet bundle) on a process tray in the apparatus are aligned and perforated collectively. With this arrangement, a conventional procedure for setting the sheet bundle in the puncher can be omitted, thereby enhancing the sheet processing ability.
However, in this apparatus, since the sheet bundle stacked on the process tray is perforated collectively, a next sheet cannot be conveyed while the sheet bundle is being perforated, with the result that it is difficult to increase a sheet conveying and processing speed of the image forming apparatus. Further, since the sheet bundle is perforated collectively, for every predetermined number of sheets, the perforating operation must be performed.
To solve this problem, there has been proposed a sheet processing apparatus in which a puncher comprising a rotatable punch and a rotatable die is provided in a sheet convey path, and a sheet conveying speed and a rotational speed of the puncher are synchronized. An example of such a puncher is shown in FIGS. 10A and 10B. In FIGS. 10A and 10B, a sheet (not shown) is conveyed through a sheet guide 13a of a puncher 13 from the above, and a rear end portion of the sheet is perforated by a punch 14 rotated in a direction shown by the arrow A and a die 15 rotated in a direction shown by the arrow B. Punch debris (punched pieces) PW produced in the punching operation is collected in a die cover 15a integrally formed with the die 15. As shown by the arrow a, the punched pieces PW are discharged into a puncher frame 25 through an opening 15e of the die cover 15a and are collected in a punch dust box (not shown) through a dust dropping hole 16 of the puncher frame 25. When the box is filled with the punched pieces PW, the punch dust box is removed from the image forming apparatus and the punched pieces PW are discarded.
With an arrangement as mentioned above, it is possible to perforate each sheet without stopping the conveyance of the sheet.
However, although a large amount of punched pieces PW discharged from the die cover 15a are dropped through the dust dropping hole 16, some of punched pieces PW are gradually accumulated in an area where the hole 16 does not exist, thereby sometimes closing the dust dropping hole 16.
Further, when the die cover 15a containing the punched pieces PW therein continues to rotate, the die cover 15a is electrically charged, with the result that the punched pieces PW are adhered to inner and outer peripheral surfaces of the die cover 15a to accumulate the punched pieces PW within the die cover 15a and the punched pieces PW adhered to the outer surface of the die cover 15a are scattered into the sheet guide 13a.
Further, when the sheet is punched by the punch die 15, the punched pieces PW may not be advanced inwardly of the die but are scattered outwardly of the die toward the sheet guide.